The present invention relates to a rare earth-iron-boron permanent magnet alloy having extremely improved heat resistance and magnetizability, and a method of producing it by a rapid quenching method. It further relates to a bonded rapid quench magnet suitable for PM (permanent magnet)-type motors capable of generating sufficient power at high temperatures, and a PM-type motor such as a stepping motor, a linear actuator, etc. for use in automobiles, office automation equipment, factory automation equipment, etc.
There are various types of permanent magnet-type motors thereinafter referred to simply as "PM-type motor"), in which permanent magnets are used for stators or rotors to generate an electromagnetic force in cooperation with current flowing through windings. Mainly from the aspects of power and cost performance, small PM-type motors generating a power up to 20 Watts use bonded ferrite magnet composed of ferrite magnet powder and binders, and those generating a power of 20-300 Watts use sintered ferrite magnets. On the other hand, large PM-type motors generating a power of 300-1700 Watts or higher use sintered ferrite magnets having high coercive forces.
As permanent magnets having better magnetic properties than ferrite magnets, rare earth-cobalt (R--Co) magnets and rare earth-iron-boron permanent magnets (R--Fe--B) are known, and these magnets have been used for PM-type motors.
The rare earth-iron-boron permanent magnet alloy having high energy product are generally produced by the following two methods:
The first method is a powder metallurgy method in which an ingot of magnet alloy is used as a starting material, and the same sintering process as in the conventional samarium-cobalt magnets is used to prepare a permanent magnet (Japanese Patent Laid-Open No. 59-46008). This method is a practical one since it can produce various shapes of magnets. In addition, since the rare earth-iron-boron permanent magnet alloy is pulverized to 3-10 .mu.m and formed in a magnetic field, crystal grains in the permanent magnets are alligned with respect to their easy magnetization directions, whereby the permanent magnets are provided with a high residual magnetic flux density. For instance, a sintered magnet of Fe.sub.77 Nd.sub.15 B.sub.8 shows a residual magnetic flux density Br of 12.1 kG, and a coercive force iHc of 7.3 kOe.
The second method is a rapid quenching method in which a thin ribbon prepared by a rapid quenching method such as a single roll method is used as it is or after heat treatment to provide a permanent magnet (Japanese Patent Laid-Open No. 59-64739). In the permanent magnet shown in the above reference, crystals of 20-400 nm in diameter are arranged isotropically, and the magnet shows a residual magnetic flux density Br of 8 kG, and a coercive force iHc of 13 kOe or so.
However, the rare earth-iron-boron permanent magnet alloys suffer from practical problems such that they are much more susceptible to temperature variation in residual magnetic flux density and coercive force than the samarium-cobalt magnets. Accordingly, part of Nd, a light rare earth element, is substituted by a heavy rare earth element such as Dy to enhance the coercive force, thereby reducing the temperature variation of magnetic properties. However, these magnets show poor magnetizability because of a large coercive force, and since they contain expensive heavy rare earth elements such as Dy, they are disadvantage in cost.
Further, the rare earth-iron-boron permanent magnet alloys prepared by a rapid quenching method are essentially isotropic, showing coercive force of several tens kOe. Accordingly, they should be magnetized in an extremely large magnetic field (100 kOe or more). As a result, usual elecromagnet methods are not used, and high-magnetic field generating apparatuses such as pulse magnetization apparatuses should be used for magnetization.
In these circumstances, magnets containing Co to improve Tc were proposed (Japanese Patent Laid-Open No. 59-64733). However, the addition of Co leads to a decrease in rectangularity of the B-H curve.
In the mean time, the bonded magnets of R--Fe--B magnet alloys include bonded sintered magnets comprising magnet powder prepared by a powder metallurgy method including pulverization and binders, and magnetically isotropic bonded rapid quench magnets comprising a magnetically isotropic magnet alloy flakes prepared by a rapid quenching method and binders (Japanese Patent Laid-Open No. 59-211549), and magnetically anisotropic bonded magnets comprising magnetically anisotropic magnet powder prepared by pulverizing hot-worked magnet and binders (Japanese Patent Laid-Open No. 63-232301).
Incidentally, in Japanese Patent Laid-Open No. 59-211549, the composition range of bonded rapid quench magnet is not described, and there are only two examples of compositions which are R=15, Fe=81, B=4 (by atomic %), R=20, Fe=76, B=4 (by atomic %).
It should be noted that there is an important morphological difference between the sintered magnet and the rapidly quenched magnet in an average crystal grain size. The sintered magnet has an average crystal grain size of 1-80 .mu.m (Japanese Patent Laid-Open No. 59-163802), while the rapidly quenched magnet has an average crystal grain size of 0.02-0.4 .mu.m (Japanese Patent Laid-Open No. 59-64739). This means that the average crystal grain size of the rapidly quenched magnet is close to about 0.3 .mu.m, which is a critical size of a single magnetic domain, which contributes to high coercive force.
In such circumstances, EP 0,242,187 was recently published, which discloses a permanent magnet having a composition represented by the formula: EQU [R.sub.a (Ce.sub.b La.sub.1-b).sub.1-a ].sub.x (Fe.sub.1-z Co.sub.z).sub.100-x-y-w.sup.B.sub.y.sup.M.sub.w
wherein R is at least one rare earth element including Y and excluding Ce and La, and M is at least one selected from Zr, Nb, Mo, Hf, Ta and W, 5.5.ltoreq.x&lt;12, 2.ltoreq.y&lt;15, 0.ltoreq.z.ltoreq.0.7, 0&lt;w.ltoreq.10, 0.80.ltoreq.a.ltoreq.1.00, and 0.ltoreq.b.ltoreq.1, the magnet consisting of a fine crystalline phase or a mixed phase of a fine crystalline phase and an amorphous phase. In Example 7 of EP 0,242,187, a thin ribbon of 9.5 Nd-8 B-4 Zr-bal. Fe prepared by a rapid quenching method shows, after an aging treatment at 700.degree. C. for 10 minutes, the following temperature characteristics: ##EQU1## in a temperature range of 20.degree.-110.degree. C.
This reference mentions:
"The structure obtained by rapid cooling depends on the cooling condition and is amorphous or is composed of a mixed phase of amorphous and fine crystals. The fine crystalline structure or the mixed phase structure of amorphous and fine crystals, and the size of the constituent phases of the structure, can be further controlled by annealing, to enhance the magnetic properties. The magnetic properties are enhanced when at least 50% of the fine crystals has a size in the range of from 0.01 to less than 3 .mu.m, preferably from 0.01 to less than 1 .mu.m. The amorphous-free structure provides higher magnetic properties than the mixed phase structure.
"The annealing of a magnet rapidly cooled and solidified by the liquid-rapid cooling method is carried out at a temperature range of from 300.degree. C. to 900.degree. C. for 0.001 to 50 hours in an inert gas atmosphere or a vacuum. The rapidly cooled magnet having a composition according to the present invention and subjected to the annealing becomes insensitive to the conditions for rapid cooling, thereby stabilizing the magnetic properties."
However, the permanent magnets are required to have a good heat resistance at a temperature of 140.degree. C. or higher for some applications such as motors disposed in automobiles. Although the permanent magnet of EP 0,242,187 shows a good heat resistance up to 110.degree. C., it fails to meet the heat resistance requirement at a temperature of 140.degree. C. or higher.